Many different types of accelerometers exist in the prior art. These devices typically measure acceleration by performing measurements on a mass which is coupled to a spring assembly or some other device. This spring assembly or other device, such as a beam will, due to its resiliency or movement compress with stress by movement of the mass. Strain gauges placed on the spring or beam will respond to the compression or stretching and produce an output voltage proportional to the same. The piezoresistive strain gauge has been utilized in such accelerometers with great success and also has been employed for measuring strain in various other transducer configurations. It is known that the piezoresistor is traditionally more sensitive than for example other types of gauges, such as metal wire or foil type strain gauges. Generally the performance of an accelerometer is determined by two quantities: 1) the output as a function of acceleration and, 2) the natural frequency of operation. It is desirable to maximize both quantities in any given design, but because the greater the mass of the seismic structure the greater the output per acceleration (g) but the lower the natural frequency.
In order to eliminate the effective mass, one frequently refers to a quantity designated as the figure of merit (FOM) of the accelerometer system. The figure of merit (FOM) is the product of the output per g times the natural frequency squared (g×f2). Since the output per “g” (acceleration) is directly proportional to the mass and the natural frequency is proportional to the square root of the system stiffness divided by the mass, the FOM will be independent of the mass. Due to the increasing demands of present technology, it is desirable to fabricate an accelerometer with high figures of merit, excellent thermal characteristics and enhanced ruggedness. It is further desirable to employ an accelerometer having improved frequency response to enable one to measure relatively high frequency, small magnitude accelerations. Thus, the high FOM and very small displacement are apparent in the shimmed beam of prior art devices. To design a mechanical stop for a rectangular mass/beam accelerometer is very difficult. With typical displacements on the order of 0.0001 inch and standard manufacturing tolerances of 0.005 inch, each transducer thus would require a custom adjusted stop mechanism. A stop mechanism is a mechanism or apparatus which limits the movement of the spring or beam in multiple directions. In this manner, the movement is limited to avoid fracturing or breakage of the beam for large magnitude acceleration or forces which would otherwise bend a thin beam beyond its mechanical limits.
Stop mechanisms exist for various transducer devices as well as for beams. These mechanisms serve to limit the displacement of the device in various directions and operate to limit movement of device to avoid breakage of the device for large forces. In most piezoresistive based accelerometers, the deflecting member is either a cantilever beam to which a seismic mass is attached, or a specially fabricated seismic mass, which in itself contains the requisite beams. Both methods have their own advantages and disadvantages. In the cantilever beam approach, a narrow section of the beam contains piezoresistive strain gauges on the top and bottom of the beam. One end of the beam is clamped to obtain a cantilever action and a seismic mass is mounted to the other end or free end of the beam. Under applied acceleration, the beam deflects, giving rise to an output from the piezoresistive bridge. In the prior art, the narrow portion of the beam could be spanned by very short sensors positioned on shims, above or below the beam. In the former case, the stiffness of the beam is determined only by the dimensions of the beam, but when the narrow portion of the beam is spanned by the short sensors on shims, they also help determine the overall stiffness and give rise to a higher FOM for the accelerometer structure. In the former case, with the larger deflection, it is possible by careful control of the beam's dimensions, to install stops to limit deflections at higher accelerations. In the latter, with the beam spanned by sensors on shims the stiffness is increased resulting in a much smaller deflection. Thus, designing stops for the much smaller deflections at high accelerations is an extremely challenging and difficult task. This results in excessive cost to such devices operating as indicated above.
Alternative techniques and devices for transducers including accelerometers adapted with a stop which further provides a higher FOM and an improved operation is desired.